Step 1

Heat oil and margarine in a small skillet or wok over medium-high heat. Add onion and sauté until very brown, 10 to 15 minutes; do not undercook or sauce will taste funny.

Step 2

Add garlic and ginger to onions; sauté for an additional 2 minutes. Transfer mixture to a food processor and blend until smooth. Do not rinse the food processor.

Step 3

Place blended onion mixture in a large saucepan. Stir in coriander, cumin, cinnamon, black pepper, cayenne pepper, and turmeric; cook over low heat until mixture is thick and has a paste-like consistency.

Step 4

Purée tomatoes, chile peppers, and cilantro in the food processor until smooth. Add to onion mixture and stir over low heat, cooking off moisture from tomatoes and peppers. Add yogurt a little bit at a time, stirring constantly to avoid curdling.

Step 5

Blend mixture in the food processor until smooth. Return to saucepan, add water, and increase heat to high; bring sauce to a rolling boil. Cover saucepan and boil for 3 to 5 minutes. Reduce heat and simmer until desired consistency is reached.